tests: fix tst_qqmlmoduleplugin::incorrectPluginCase()

It gives different error message based on the case sensitivity of
the file system on Mac.

Task-number: QTBUG-32652
Change-Id: I52415126e63978c9f80b7652e0116e0e07703fd8
Reviewed-by: Liang Qi <liang.qi@digia.com>